(First Conventional Example)
Optical encoders are roughly classified into two types based on the difference in their position detection methods: an incremental type and an absolute type (an absolute position detecting type). The configuration and operation of a conventional incremental type encoder are described below. As shown in FIG. 49, a conventional incremental type encoder comprises a light source 501; a collimator lens 502 for making beams from the light source 501 parallel; a movable plate 503 that rotates around a shaft 512; a fixed plate 506 disposed opposite to the movable plate 503; and a light receiving device 509 with two light receiving parts 510 and 511.
The movable plate 503 has an A/B phase signal region in which slits or diffraction grating 504 is equal pitch on its circumference; and a Z phase signal region in which only one slit 505 is disposed on its circumference. Similarly, the fixed plate 506 has an A/B phase signal region in which slits or diffraction grating 507 is disposed at the same pitch as in the movable plate; and a Z phase signal region in which only one slit 508 is disposed on its circumference. The light receiving part 510 of the light receiving device 509 detects light that has been transmitted through the diffraction grating 504 of the movable plate 503 and the diffraction grating 507 of the fixed plate 506. The light receiving part 511 of the light receiving device 509 detects light that has been transmitted through the slit 505 of the movable plate 503 and the slit 508 of the fixed plate 506.
By detecting light that has been transmitted through the A/B phase signal regions of the moving and fixed plates 503 and 506 (the diffraction gratings 504 and 507), a signal depending on the rotating angle of the movable plate 503 (an A/B phase signal) is detected, and by detecting light that has been transmitted through the Z phase signal regions (the slits 505 and 508), a signal representing the origin of the movable plate 503 (a Z phase signal) is detected. Generally, an output signal from the light receiving part 509 is binarized into a pulse signal, which is then processed to detect the position. To process the signal easily, the Z phase signal is desirably synchronized with the A/B signal. Thus, the pulse of the Z phase signal must be synchronized with only one pulse of the A/B phase signal.
Next, a method for creating a light shielding pattern such as slits is described with reference to FIG. 50. As shown in FIG. 50A, a photo resist 522 is applied to the surface of a transparent substrate 521. Then, a mask 523 with a specified pattern which has been produced by electron beam exposure is adhered to the surface of the photo resist 522 or allowed to approach the photo resist 522 as in FIG. 50B. The substrate is then irradiated with light of a wavelength region that can be responded by the resist in order to make only the exposed resist soluble or insoluble, and the mask is then removed. When the substrate 521 is then immersed in a resist solvent, a mask pattern 522' from the resist is transferred to the surface of the substrate 521 as in FIG. 50C. The substrate 521 with the mask pattern 522' transferred thereto is installed in a deposition apparatus (not shown) to deposit a metal 524 such as chrome thereon as in FIG. 50D. The substrate 521 is subsequently removed from the deposition apparatus, and an organic solvent such as acetone is used to remove the photo resist 522' remaining on the substrate 521 as in FIG. 50E. As a result of this series of operations, a light shielding pattern 525 such as slits is formed on the substrate 521.
As described above, a large number of operations are required to produce the light shielding pattern 525 such as slits on the substrate 521. In addition, the mask 523 and the substrate 521 cannot be aligned with each other easily. Thus, as is well known, high costs are required to form a pattern such as slits on the moving and the fixed plates 503 and 506. Consequently, incremental type optical encoder uses phase type optical elements in the A/B phase signal regions (the diffraction gratings 504 and 507) of the moving and the fixed plates 503 and 506 to reduce costs.
Phase type optical elements can be produced by forming recesses and convexes on the surface of the substrate. A phase type optical element manufacturing process is shown in FIG. 51. A transparent resin 532 such as acryl or polycarbonate which is made flowable by heating is poured into a mold 531 of a specified shape and solidified therein. The phase type optical element has the shape of the mold transferred thereto. Compared to the method for producing a light shielding pattern such as slits on the surface of the substrate, this method does not require the substrate and the pattern to be aligned with each other or require operations such as application of a photo resist, ultraviolet radiation, development, deposition of a metallic film, and washing thereof.
As one example in which the A/B phase signal region comprises a phase type diffraction grating, which is one of the phase optical elements, a conventional incremental type encoder shown in, for example, JPA6-042981 is known. The configuration of this encoder is shown in FIG. 52. In this figure, the light source 501 comprises a semiconductor laser or a relatively coherent light emitting diode. Beams from the light source 501 are made parallel by the collimator lens 502 and are incident on the movable plate 503. The movable plate 503 has a phase type diffraction grating 534 that mainly generates .+-.1 order diffracted light, and is disposed approximately perpendicularly to the optical axis of the parallel beams so that it can be rotated around a rotation center 512 parallel to the optical axis. The fixed plate 506 has a phase type diffraction grating 537 with a grating pitch P equal to that of the phase type diffraction grating 534 on the movable plate 503, and is disposed approximately perpendicularly to the optical axis. The light receiving part 510 receives light formed based on the relative locational relationship between the phase type diffraction gratings 534 and 537. Beams from the light source 501 are made parallel by the collimator lens 502 and are then approximately perpendicularly incident on the movable plate 503. The light incident on the movable plate 503 is diffracted into a +1 order diffracted beam and a -1 order diffracted beam by the phase type diffraction grating 534 on the movable plate 503. The light is incident on the phase type diffraction grating 537 of the fixed plate 506 and diffracted into a +1 order diffracted beam and a -1 order diffracted beam, respectively. Since the phase type diffraction gratings 534 and 537 have an equal grating pitch P, the diffraction angles of the phase type diffraction gratings 534 and 537 are equal. Consequently, the optical path of the light diffracted into a -1 order beam by the phase type diffraction grating 534 and then into a +1 order beam by the phase type diffraction grating 537 ((-1, +1) order diffracted light) becomes equal to the optical path of the light diffracted into a +1 order beam by the phase type diffraction grating 534 and then into a -1 order beam by the phase type diffraction grating 537 ((+1, -1) order diffracted light), so these optical paths interfere with each other, varying the intensity of the light. Since the interference condition depends on the movement .delta. of the movable plate 503, the intensity of the light varies depending on the movement .delta. of the movable plate 503. That is, since the movement .delta. of the movable plate 503 varies the amount of light received by the light receiving part 510, the movement of the movable plate 503 can be detected.
On the other hand, methods for providing recesses and convexes on the surface instead of slits have been examined for the Z phase signal region, as in the A/B phase signal region. As shown in FIG. 53, a condensing lens 541 is provided on the movable plate 503 so that a condensed spot can be received by the light receiving part 511 to detect the movement reference point of the movable plate 503. In this case, the accuracy in detecting the Z phase is substantially determined by the sizes of the condensed spot and the light receiving part 511, and increases as the sizes decrease.
(Second Conventional Example)
An incremental type optical encoder shown in FIG. 49 detects the position of the movable plate 503 based on the movement of the A/B phase signal relative to the Z phase signal as a reference. Thus, position detection cannot be carried out while power is being supplied, and it is essential to detect the position of the reference. On the contrary, absolute type optical encoders that can detect a current position at any time from any position depending on the different slit patterns are known.
The configuration and operation of absolute type optical encoders are described. As shown in FIG. 55, a conventional absolute type optical encoder comprises a light source 601; a collimator lens 602 for making beams from the light source 601 parallel; a movable plate 603 rotating around a shaft 612 and having a plurality of slit tracks with slits 604 disposed at approximately equal pitch on its circumference; a fixed plate 606 disposed opposite to the movable plate 603 and having a plurality of slits 607 corresponding to the plurality of slit tracks 604 on the rotating plate 603; and a light receiving device 609 having a plurality of light receiving parts 610 corresponding to the plurality of slits 607. The slits 604 are installed at a different pitch in each slit track on the rotating plate 603. Each light receiving part 610 detects light transmitted through a slit 604 of the movable plate 603 and a slit 607 of the fixed plate 606. Based on the pattern of a detection signal from the light receiving part 610, the absolute position of the rotating plate 603 can be detected.
(Third Conventional Example)
The position of an object has been commonly detected in a non-contact manner by irradiating the object with light, projecting its image onto a television camera, and binarizing an output signal from a linear array sensor to detect the position, or forming a slit in a moving object, transmitting light from the light source through the slit in such a way that the light is incident on a light receiving part, and binarizing the output signal from the light receiving part to detect the moving reference point of the moving object. A conventional position detection method shown in, for example, JPA2-44202 is described with reference to FIG. 56.
FIG. 56 shows a plan view of a position detection apparatus that is a third conventional example. In this figure, 701 is a light source; and 702 is a moving object on which a slit 703 is formed. Reference numeral 705 designates a light beam that has passed through the slit 703, and 704 is a light receiving part. The moving object 702 is disposed between the light source 701 and the light receiving part 704, and moves perpendicularly to the shaft which couples the light source 701 and the light receiving part 704. In response to the movement of the moving object 702, the light beam 705 also moves. AA is the distance between the light source 701 and the moving object 702, and B is the distance between the moving object 702 and the light receiving part 704. In addition, .DELTA. is the movement of the moving object 702, and .delta..delta. is the movement of the light beam 705. In this case, the movement .delta..delta. of the light beam 705 can be expressed as the following Equation (1): ##EQU1##
The incremental type encoder that is the first conventional example is disadvantageous in that if the Z phase signal region has recesses and convexes on the surface instead of slits, the Z phase detection accuracy cannot be improved easily. The diameter of the condensed spot is determined by the size of the light source 501 and the focusing distance of the collimator lens 502 in geometrical optics. As shown in FIG. 54, if the size of the light source 501 is referred to as .phi.s1, the focusing distance of the collimator lens 502 is referred to as fs1, and the focusing distance of the condensing lens 541 installed on the Z phase signal region is referred to as fs2, then the diameter .phi.s2 of the condensed spot 551 can be expressed as the following Expression (2) based on the Gauss's formula in geometrical optics. ##EQU2##
Thus, to reduce the diameter .phi.s2 of the condensed spot 551, firstly the size .phi.s1 of the light source 501 may be reduced, secondly the focusing distance fs1 of the collimator lens 502 may be increased, or thirdly the focusing distance fs2 of the condensing lens 541 on the Z phase signal region may be reduced. To reduce the size of the first light source 501, however, a light shielding part such as a pin hole may be formed near the light source 501, but this disadvantageously reduces the amount of available light. In addition, since the sizes of the A/B and Z phase signal regions determine the diameter of required parallel beams, that is, the diameter of the collimator lens 502, increasing the focusing distance fs1 of the second collimator lens 502 reduces the use efficiency of light from the light source 501. In some methods for generating signals in the A/B phase signal region, the distance between the fixed plate 506 and the light receiving part 511 must be increased to separate unwanted diffracted light from, for example, the fixed plate 506. This prevents the focusing distance fs2 of the condensing lens 541 on the Z phase signal region from being significantly reduced. Alternatively, the light receiving part 510 for the A/B phase signal may be separated from the light receiving part 511 for the Z phase signal so that the light receiving part 510 for the A/B phase signal can be installed at a desired distance from the fixed plate 506, while the light receiving part 511 for the Z phase signal can be installed closer to the fixed plate 506. This embodiment, however, involves a complicated structure and requires accurate assembly, thereby increasing costs.
The light source 501 of the optical encoder normally comprises a light emitting diode, and the light emitting diameter .phi.s1 of the light emitting diode is not smaller than 100 .mu.m. In addition, the focusing distance of the collimator lens 502 must be about 5 mm or larger depending on the size of the product or the specification, and the distance between the movable plate 503 and the light receiving part 511 must be about 20 mm or larger. As a result, the diameter of the light spot of the condensing lens 541 installed on the movable plate 503 can be calculated as about 400 .mu.m, using Equation (2). Compared to the A/B phase signal with a pitch (or a period) of 10 .mu.m, the Z phase signal is wide. To increase the accuracy of Z phase detection, the size of the light receiving part 511 may be reduced compared to the light spot diameter to set a higher threshold used in binarizing a detected signal. This encoder, however, is likely to be subjected to electric noise or the variation of the light intensity of the light source 501, so it cannot detect the Z phase stably.
Furthermore, this method cannot easily synchronize the Z phase signal with the A/B phase signal used to detect the movement of the movable plate 503. In the method shown in FIG. 53, the A/B phase signal has a wave form determined by the relative locational relationship between the movable plate 503 and the fixed plate 506, whereas the Z phase signal can be generated without the use of the fixed plate 506 and has a wave form determined by the relative locational relationship between the movable plate 503 and the light receiving part 511. Thus, to synchronize the A/B phase signal with the Z phase signal, the elements in the movable plate 503, fixed plate 506, and light receiving part 511 must be positioned very accurately. Furthermore, a slight offset among these elements may prevent the A/B phase signal and the Z phase signal from being synchronized.
In addition, in the absolute type encoder that is the second conventional example, a plurality of slit tracks are formed on the moving and the fixed plates 603 and 606, and the slits 604 are disposed at a different pitch in each slit track. This prevents the use of phase type optical elements with recesses and convexes at an equal pitch on the surface of the substrate, as in the incremental type encoder of the first conventional example, and requires the deposition of a thin film of metal on a transparent substrate to form a slit as shown in FIG. 50. Consequently, costs cannot be reduced easily.
In the position detection method that is the third conventional example, to improve the accuracy in detecting the moving reference point of the moving object 702, the movement .delta..delta. of the light beam 705 may be increased using the movement .DELTA. of the moving object 702. Specifically, the distance B may be increased to reduce the distance AA. When, however, the distance B is increased while the distance AA is simultaneously reduced, the diameter of the light beam 705 is increased on the light receiving part 704. This reduces the accuracy in detecting the moving reference point of the moving object 702. On the other hand, if the size of the slit 703 is excessively reduced to reduce the diameter of the light beam 705, the light may be diffracted, thereby increasing the diameter of the light beam 705 on the light receiving part 704. In addition, if the size of the slit 703 is reduced, the amount of light received by the light receiving part 704 may be reduced, thereby enhancing the effects of noise and reducing the accuracy in detecting the moving reference point. Furthermore, to prevent the effects of diffraction, the distance B between the slit 703 and the light receiving part 704 may be reduced. When this distance is reduced, however, the moving object 702 may contact with the light receiving part 704 and both may be damaged.